The present invention relates to a process for producing a carboxylic acid ester by reacting an aldehyde and an alcohol in a liquid phase in the presence of molecular oxygen by using a catalyst and to a catalyst used for the process.
Catalysts which have been proposed to be used in a process for producing a carboxylic acid ester from an aldehyde and an alcohol in the presence of molecular oxygen by using a catalyst include, for example, a palladium-lead type catalyst disclosed in JP-B-57-35856, JP-B-4-72578, JP-A-57-50545 and others, a palladium-tellurium type catalyst disclosed in JP-A-61-243044, a palladium-thallium-mercury type catalyst disclosed in JP-B-57-35860, a palladium-alkaline earth metal-zinc-cadmium type catalyst disclosed in JP-B-57-19090, and a palladium-bismuth type catalyst disclosed in JP-B-61-60820, JP-B-62-7902, JP-A-5-148184 and others. As to the carrier of the catalyst used for such processes, there have been proposed, for example, calcium carbonate in JP-B-57-35856 and JP-B-57-35860, zinc oxide-alumina, titania-lanthanum oxide and zinc oxide-titania in JP-B-4-46618, zinc oxide in JP-B-4-72578, a carrier having a specific surface area of not more than 70 m2/g in JP-A-57-50942 and a hydrophobic carrier in JP-A-5-148184.
However, these catalysts are apt to differ in the yield of carboxylic acid esters even when the components and/or the carrier of the catalyst are of the same composition. Therefore, the development of a process, improved in the above-mentioned point, which can produce carboxylic acid esters with a high yield has been eagerly awaited.
The object of the present invention is to provide a process for producing a carboxylic acid ester from an aldehyde and an alcohol with a high yield and a catalyst used for the process.
Thus, the present invention provides a process for producing a carboxylic acid ester, comprising reacting an aldehyde and an alcohol in a liquid phase in the presence of molecular oxygen by the use of a catalyst comprising at least palladium and X (X represents bismuth and/or lead) supported on a carrier, wherein the catalyst used has an acid strength, pKa, of more than 4.8 and shows an ammonia chemical adsorption amount at 0xc2x0 C. of 0-150 xcexcmol/g-catalyst.
The present invention further provides a catalyst used for producing a carboxylic acid ester by reacting an aldehyde and an alcohol in a liquid phase in the presence of molecular oxygen, which comprises at least palladium and X (X represents bismuth and/or lead) supported on a carrier, has an acid strength, pKa, of more than 4.8 and shows an ammonia chemical adsorption amount at 0xc2x0 C. of 0-150 xcexcmol/g-catalyst.
In the catalyst and the process according to the present invention, the aldehyde used as a starting material may be, for example, aromatic aldehydes, such as benzaldehyde, methylbenzaldehyde, and nitrobenzaldehyde, saturated aliphatic aldehydes, such as acetaldehyde, propionaldehyde and isobutyl aldehyde, and unsaturated aliphatic aldehydes, such as acrolein, methacrolein and crotonaldehyde. The alcohol of a starting material may be, for example, methanol, ethanol, isopropanol, allyl alcohol and methallyl alcohol.
The catalyst of the present invention and the catalyst used in the process of the present invention comprise palladium, as a catalyst component, supported on a carrier and additionally bismuth and/or lead, as a catalyst component(s), supported on the carrier. The term xe2x80x9ccatalystxe2x80x9d herein refers not only to the catalyst components supported on a carrier but also to the whole catalyst system including the carrier. A starting material for palladium used in preparing the catalyst may be, for example, palladium acetate, palladium chloride, palladium nitrate, palladium ammonium chloride, and palladium-ammine complex salt, a starting material for bismuth may be, for example, bismuth acetate, bismuth carbonate, bismuth chloride, bismuth nitrate and bismuth sulfate, and a starting material for lead may be, for example, metal compounds, such as lead acetate, lead carbonate, lead chloride, lead nitrate, lead sulfate, lead tartrate and lead citrate. Besides palladium, bismuth and lead, third components, such as chromium, iron, cobalt, zinc, barium and silver, may be supported as catalyst components on the carrier. The catalyst components are present on the carrier in the form of a metal and/or a metal compound.
In the catalyst of the present invention and the catalyst used in the process of the present invention, the amounts of the respective catalyst components to be supported on the carrier are, based on 100 parts by weight of the carrier, preferably 1-15 parts by weight, more preferably 3-13 parts by weight for palladium, and 0.1-15 parts by weight, more preferably 0.5-12 parts by weight for X. When the catalyst component is a metal compound , the above-mentioned amount to be supported is calculated in terms of the weight of the metal atom in the metal compound. The carriers may be for example, calcium carbonate, zinc oxide, silica and silica-magnesia. Average particle diameter and specific surface area of the carrier are, for example, 5-150 xcexcm and 50-200 m2/g, respectively.
The catalyst of the present invention and the catalyst used in the process of the present invention have an acid strength, pKa, of more than 4.8. The acid strength, pKa, herein is an index which indicates the degree of acidity of the surface of a material. It is signified that the larger the value of pKa, the weaker the acidity. The acid strength pKa is determined according to the method described in Shokubai (catalyst), vol. 11 pp. 210-216 (1969) (written by Isao Matsuzaki et al., published by Shokubai Gakkai (Catalyst Society)) by using an indicator which changes its color in a predetermined range of pKa. When a catalyst which has a pKa of not more than 4.8 is used, by-products such as acetals tend to be formed markedly to lower the yield of the carboxylic acid ester of the objective product.
The catalyst of the present invention and the catalyst used in the process of the present invention show an ammonia chemical adsorption amount at 0xc2x0 C. (hereinafter referred to simply as xe2x80x9cammonia chemical adsorption amountxe2x80x9d) of 0-150 xcexcmol/g-catalyst, preferably 30-140 xcexcmol/g-catalyst. The term xe2x80x9cammonia chemical adsorption amountxe2x80x9dherein refers to the amount of ammonia chemically adsorbed to 1 g of catalyst at 0xc2x0 C. It is signified that the larger the value of the above-mentioned amount, the larger the amount of acid sites per 1 g of the catalyst (hereinafter referred to as xe2x80x9cacid amountxe2x80x9d). To determine the ammonia chemical adsorption amount, the total adsorption amount, which is the sum of the chemical adsorption amount and the physical adsorption amount of ammonia, and the physical adsorption amount of ammonia are determined at 0xc2x0 C. by using a common adsorption-desorption apparatus available on the market, and the xe2x80x9cammonia chemical adsorption amountxe2x80x9d can be obtained from the difference of the two amounts determined above. When a catalyst which shows an ammonia chemical adsorption amount of more than 150 xcexcmol/g-catalyst is used, by-products, such as acetals, tend to be formed markedly, to lower the yield of the objective product.
The catalyst of the present invention and the catalyst used in the process of the present invention can be prepared by conventional methods. As an example of the method of preparation, the preparation of a catalyst comprising palladium, bismuth and iron supported on a silica-magnesia carrier is described below. First, palladium chloride, bismuth nitrate and nitric acid are added to water, and the resulting mixture is heated to form a solution. Then, silica-magnesia powder, and thereafter a reducing agent such as formalin are added to the solution, and the resulting mixture is stirred with heating for a predetermined time. Thereafter, the mixture is filtered and the solid obtained is immersed in an aqueous solution of ferric nitrate. In this time, if desired, the solid may be reduced again with a reducing agent to deposit a metal. The solid is again collected by filtration, and then dried to obtain a catalyst. The catalyst obtained may also be activated by conventional methods.
In the present invention, a carboxylic acid ester is produced by reacting an aldehyde and an alcohol in a liquid phase in the presence of molecular oxygen by using a catalyst which has an acid strength, pKa, of more than 4.8 and shows an ammonia chemical adsorption amount of 0-150 xcexcmol/g-catalyst. The molar ratio of the aldehyde to the alcohol of the starting materials is preferably 1:100 to 1:1, more preferably 1:80 to 1:3.
In carrying out the reaction according to the process of the present invention, the catalyst is dispersed as a suspension in the liquid phase. The reaction may be conducted either batch-wise or semibatch-wise or continuously. The source of the molecular oxygen used as the oxidizing agent may be air, oxygen-enriched air, oxygen or the like. The reaction temperature is preferably 0-100xc2x0 C., more preferably 30-80xc2x0 C. The reaction may be conducted at ordinary pressure or under applied pressure.
The molecular oxygen is used in an amount sufficient to form the intended carboxylic acid ester. The amount is preferably 10-500 ml/min relative to 100 ml of the reaction liquid. As to a solvent used for forming the liquid phase, for example, the aldehyde and/or the alcohol used in the process of the present invention may be used as such to form the liquid phase, but the solvent is not limited thereto. For example, hexane, acetone, benzene and the like may be used as the solvent. The amount of the catalyst to be used is not particularly limited, and may be, for example, 0.01 g-1 g per 1 g of aldehyde.